This invention relates to a method and system for medical imaging. More particularly, this invention relates to a method and system for extended volume imaging using a Magnetic Resonance Imaging (MRI) system and employing a moving patient table.
Certain clinical situations require that a head-to-toe scan of a patient be made. For example, metastatic cancer can occur anywhere in the body and patients at risk for metastatic disease need to be evaluated regularly. Currently head-to-toe or, alternatively, extended volume imaging is generally performed with whole body positron emission tomography (PET) systems or nuclear studies in which small amounts of radioactive substances are given to the patient and allowed to collect in regions of rapid tumor growth. The imaging sensitivity and specificity of magnetic resonance (MR), however, makes MR a more desirable choice for diagnosis particularly when Gadolinium (Gd) contrast agents are employed. Gd contrast agents are typically administered intravenously and tend to collect in regions of angiogenesis associated with active tumors. Unfortunately, MR scanners have limited sensitive volumes and whole body scanning requires that a series of images be made at different stations. Patient motion and table registration can make the joining of these separate images a challenge and tend to create image artifacts referred to as xe2x80x9cstitching artifactsxe2x80x9d.
Generally, imaging using a MRI system involves imaging a volume of interest in a MRI scanner""s usable volume. The usable volume is defined as a contiguous area inside the patient bore of a Magnetic Resonance scanner and it can be limited in size. Typically, when the usable volume fails to cover an extended object, a method for examining the whole volume containing the object employs repeated executions of positioning and imaging a fraction of the whole volume within the scanner""s usable volume to obtain regional images. A subsequent assembling operation then assembles or xe2x80x9cstitchesxe2x80x9d the regional images together to produce a final image of the whole volume of interest. Such an approach is typically challenged by the xe2x80x9cstitchingxe2x80x9d artifact issue wherein resulting final images often suffer from distinctive artifacts at the boundaries of the xe2x80x9cstitchedxe2x80x9d pieces.
Existing techniques achieve correct combination of regional images through full spatial encoding along patient table motion direction. With other existing methods, the patient table is held stationary while data is collected and moved between the collection of the regional images. These techniques minimize xe2x80x9cstitchingxe2x80x9d artifacts by using slab selection profiles that are as rectangular as possible, and/or discarding image data near the boundaries. As a result, these techniques tend to be inflexible, require prolonged radio frequency (RF) excitation, and involve considerable acquisition efficiency degradation.
What is needed is a method and system for extended volume imaging, such as head-to-toe imaging, using a MRI system in order to benefit from the specificity and image sensitivity of a MRI system. What is further needed is a method and system that is sufficiently fast to minimize image artifacts caused by patient motion.
In a first aspect, an imaging apparatus for producing Magnetic Resonance (MR) images of a subject is provided. The apparatus comprises a magnet assembly for producing a static magnetic field, a gradient coil assembly for generating a magnetic field gradient for use in producing MR images and at least one radiofrequency (rf) coil assembly for transmitting a radiofrequency pulse and for detecting a plurality of magnetic resonance (MR) signals induced from the subject. The apparatus further comprises a positioning device for supporting the subject and for translating the subject into the magnet assembly and a plurality of receivers for receiving the plurality of MR signals. Each of the receivers is adapted to be adjusted in phase or frequency changes in response to translation of the positioning device.
In a second aspect, a method for producing an image from an extended volume of interest within a subject using a Magnetic Resonance Imaging (MRI) system where the extended volume of interest is larger than an imaging portion of a magnet within the MRI system is provided. The method comprises the steps of: translating the extended volume using a positioning device along an axis of the MRI system through the imaging portion of the magnet wherein the positioning device is translated continuously at a plurality of incremented positions along the axis; detecting a plurality of MR signals from at least one radiofrequency (RF) coil for a given field-of-view within the MRI system as the positioning device is translated; and, sending the plurality of MR signals to a plurality of receivers. Each of the receivers are configured to be adjusted in at least one of phase and frequency in response to the positioning device being translated. The method further comprises computing a plurality of respective sub-images corresponding to the plurality MR signals for each of the plurality of receivers and for the given field-of-view (FOV) at each of the incremented positions and combining the plurality of respective sub-images to form a composite image of the volume of interest.